I have tried the following:
ssh-add -K ~/.ssh/id_rsa
This seems to work until I end my terminal session, but then I get prompted for my passphrase again with a new session.
Here is what I see after running the command above:
Identity added: /Users/xxx/.ssh/id_rsa (/Users/xxx/.ssh/id_rsa)
I am on iTerm2 Build 3.0.13
with zsh: stable 5.3.1
running on macOS Sierra 10.12.2